Mantis Tiller Honda GX25 cuts Out @ High RPM

[mowandgo]

Mantis Tiller Honda GX25 cuts Out @ High RPM. It starts & runs well until revs get high and it just cuts out restarts ok. I did think it might be kill switch wire earthing on body. I checked all the obvios. Exhaust, Plug, clean out carb and tank breather. I am mythed about it wonderd if you guys could help.

[Stubby]

Mantis Tiller Honda GX25 cuts Out @ High RPM. It starts & runs well until revs get high and it just cuts out restarts ok. I did think it might be kill switch wire earthing on body. I checked all the obvios. Exhaust, Plug, clean out carb and tank breather. I am mythed about it wonderd if you guys could help.

 

Coil breaking down at high reves ?

[mowandgo]

I did think thats what it was likely to be. However I have no way of testing it & there too expensive to buy incase my diagnostics is wrong I would have two good coils and still an ill engine.

[Gnome]

Are you accidentally touching the switch when holding the handles as I have done this a few times, the switch is in an easy position to to this. perhaps the slightly higher vibrations at full revs might be rocking the switch.

[GardenKit]

Take the kill wire off the coil and try it. If this cures it then the issue lies with the kill circuit.

[mowandgo]

Are you accidentally touching the switch when holding the handles as I have done this a few times, the switch is in an easy position to to this. perhaps the slightly higher vibrations at full revs might be rocking the switch.

 

I have done that a few times. When this fault started I thought that is what was happening but I was not that lucky.

[mowandgo]

Take the kill wire off the coil and try it. If this cures it then the issue lies with the kill circuit.

 

I have disconnected all wires from coil still behaves the same. I was told to ohm test the coil but not sure what ohms it should and should not say. Tried a different plug too. I can work with it runs on 3/4 throttle or under load fine just full rpm it shuts it self off. Restarts with little effort. Makes me swear lol.

[GardenKit]

I cant really see it being the coil as it starts straight away. A faulty coil usually dies when hot and wont restart until it cools. Testing it cold will not be of much help I fear.

 

I would run it with my spark tester in line and watch the spark. I would try to see if the spark failed just before the engine died, although it all happens so fast its difficult to see.

 

I tend to think its more fuel related though, and a thought that comes to mind is fuel vapourisation. This is caused by heat and can be accelerated by vibration. If you are running fresh fuel it should not be a problem, but its just possible that if you are running an ethanol rich fuel (Tesco type) and it was bought some months back (i.e. winter grade) that you are getting vaporisation.

 

Winter grade fuels are mixed to vaporise more readily to aid starting in cold conditions, but can cause these issues if used in the warmer weather.

 

I would change the fuel for new, non supermarket, fuel and try again. Its easy to do and worth a try.

 

The other thing that maybe worth checking is the exhaust filter. I have seen the odd example (in general, not necessarily on a Honda GX25) of carbon crusted filters which get forced harder together by increased exhaust flow at high speed and then suddenly block of the exhaust port completely. Again, this is a quick ,easy check and costs nothing.

 

I always like to eliminate the simple things first.

[mowandgo]

Hi Garden Kit I have run it with exhasut complety off. I cleaned the carb out with compressed air. I usaly only buy BP, Shell or Esso I try to avoid other fuels as I have noticed a difference in quality before. Its not my bread & butter tool like a strimmer or blower where its used on almost every job. It does spend some time sitting around. Should I flush the fuel? Do you think I should try some fuel cleaner? I also had a supision it might be valves jumping but do not know if that would be possible.

[GardenKit]

Hi Garden Kit I have run it with exhasut complety off. I cleaned the carb out with compressed air. I usaly only buy BP, Shell or Esso I try to avoid other fuels as I have noticed a difference in quality before. Its not my bread & butter tool like a strimmer or blower where its used on almost every job. It does spend some time sitting around. Should I flush the fuel? Do you think I should try some fuel cleaner? I also had a supision it might be valves jumping but do not know if that would be possible.

Hi Mow, it looks like you are one step ahead of me and have tried just about everything. I suppose it could be down to valve clearances, but I would have thought they would also effect starting. As the cam wears the clearances will increase so the valves would not be open for long. I suppose this may mean they are not open for long enough to allow top rev fuel through, but I would have thought it would just stop full revs being reached.

 

I don't see many of these so am not an authority on them, but I dis buy a secondhand one in last week which I intend to service ready for putting on sale tomorrow. I will have a good check and see if I can locate any problem areas.